Speed, distance and time

A snail, a cyclist, a jet and a rocket are all moving. But they are not moving at the same speed. How can we say exactly how fast something goes?

What is speed?

Speed is how far something travels in a certain amount of time. It answers the question: "How much distance do you cover every second?"

Look at this runner:

She runs 100 metres in 10 seconds. How far does she go every second? Divide the distance by the time:

100 metres ÷ 10 seconds = 10 metres every second

We write that as 10 m/s, and say "10 metres per second". That is her speed.

The speed rule

Here is the rule that links the three ideas:

speed = distance ÷ time

You can turn it round to find the other two:

• distance = speed × time
• time = distance ÷ speed

A triangle can help you remember all three. Cover up the one you want to find, and what you can still see tells you what to do.

• Cover distance: you see speed next to time, so multiply them.
• Cover speed: you see distance over time, so divide.
• Cover time: you see distance over speed, so divide.

Three worked examples

1. Find the speed. A cyclist rides 90 metres in 15 seconds. Speed = 90 ÷ 15 = 6 m/s.

2. Find the distance. A car drives at 20 m/s for 30 seconds. Distance = 20 × 30 = 600 metres.

3. Find the time. How long does it take to walk 300 metres at 1.5 m/s? Time = 300 ÷ 1.5 = 200 seconds.

Kilometres per hour

On the road, you will see speeds in kilometres per hour (km/h). It is the same idea with different units. To switch between them:

• multiply m/s by 3.6 to get km/h
• divide km/h by 3.6 to get m/s

So 10 m/s is 36 km/h. In Launch Lab, a launch speed of 20 m/s is 72 km/h, about as fast as a car on a country road.

How fast are real things?

Notice how big the numbers become. That is why the Orbit Keeper game uses kilometres per second.

Average speed and speed right now

The speedometer in a car shows your speed at this moment. But on a long trip, you speed up and slow down. If you divide the total distance by the total time, you get your average speed. A car that drives 120 km in 2 hours has an average speed of 60 km/h, even if it was sometimes going faster and sometimes slower.